This invention relates to the production of prilled ammonium nitrate. In this process ammonium nitrate is sprayed or injected in a flowable molten state into a stream of cooling gas, such as air, either cocurrently or countercurrently for the purpose of solidifying the droplets into small solid particles. This invention more specifically relates to preventing formation of extremely fine solid particles which are emitted with the cooling gas stream and which can pollute the surrounding environment and result in substantial product losses.
The principle of prilling or shotting molten fluids into spherical or spheroidal shapes by allowing droplets to solidify while falling is well known. Many adaptations have been used successfully to employ the surface tension effect of the molten drop and gravity to produce substantially spherical, free flowing and non-agglomerated salt particles. Most of the methods commercially used employ a mechanical device to disperse the molten salt into a cooling chamber where the molten droplets fall and cool until fusion occurs. The hot particles may cool further as they fall, depending on the height of the cooling chamber. The cooling chamber may vary in size and shape depending on the salt composition. Spray dryers and prill towers are examples of various shapes common to the art. The prilling process has become commercially widespread in recent years as a method for making small spheroidal particles from various salts and mixtures. Prilling of ammonium nitrate was first described by Williams et al in U.S. Pat. No. 2,402,196.
When ammonium nitrate is prilled, the molten salt usually contains from 0.10% to 5% water, by weight. A relatively dry melt, i.e., 99.9% ammonium nitrate, results in a dense prill, whereas a wetter melt, i.e., 95% ammonium nitrate, produces a lower density prill. Since the object of prill towers is to solidify the liquid droplets after a spherical shape has formed, the ammonium nitrate melt must enter the prill tower at a temperature above the melting point. This temperature may range from 125.degree. to 200.degree. C, depending upon the solution concentration and the particular operating characteristics of the prill tower.
Prilling technique has also been employed to granulate salt mixtures and slurries. For example, ammonium nitrate and limestone blends are prilled for the production of a fertilizer material commonly called nitrochalk, nitrolime or ammonium nitrate-limestone. This composition usually contains 55-60 wt% NH.sub.4 NO.sub.3 and about 40-45% limestone.
Although some prill towers use cocurrent flow, most commercial ammonium nitrate prilling plants use ambient air flowing upward countercurrent against the downward flow of prills. Natural draft, and forced or induced fans and blowers, are used to obtain a flow of air. The prill tower air stream has several purposes. The ammonium nitrate droplet must be free falling in the air in order to form a sphere. In addition, the air must remove both sensible heat and the heat of fusion (crystallization) of the ammonium nitrate, and the air stream may provide some drying of the ammonium nitrate particles. Prill towers are operated commercially on a once-through basis, i.e., the air is vented after passing through the prill tower. In practice, the use of ambient air makes the prilling technique subject to weather variations, often to the detriment of the operation and product.
The prill tower air stream vent has been given considerable attention as a prime source of air pollution since both government and industry have become aware of the environmental effects of particulate emissions. The prill towers vent air containing water vapor and ammonium nitrate dust (usually in submircon size) which is visible as a plume from the prill tower. In view of the possible pollution resulting from such prilling operations, effort has been expended toward reducing or eliminating the level of the emission. Most proposals have involved scrubbing or filtering systems to cleanse the air being vented. Since the emission particles are in the sub-micron range, great difficulty is encountered in their recovery.
Considerable research over recent years has documented the instability of ammonium nitrate. Cawthorn and Taylor have reported in "Kinetics of the Thermal Decomposition of Ammonium Nitrate", Report Control No. OSR-TN-54-334, U.S. Department of Commerce, Office of Technical Services, that ammonium nitrate thermally decomposes as a function of time, temperature and concentration. Dissociation and ionization of ammonium nitrate are known to vary with temperature according to the reactions: EQU NH.sub.4 NO.sub.3 .revreaction. NH.sub.3 + HNO.sub.3 EQU nh.sub.4 no.sub.3 .revreaction. nh.sub.4.sup.+ + no.sub.3.sup.-
prill tower emissions originate largely as a result of these chemical reactions. In the molten state 95% to 99.9% ammonium nitrate tends to dissociate at the point of introduction into the prill tower. Reassociation, or neutralization, occurs rapidly in the air stream forming a chemical smoke of submicron ammonium nitrate particles. The rate of particulate formation or aerosol emission is therefore a function of temperature and it has been found that the emission rate will increase as the prilling temperature increases.